This invention relates to thermoplastic molding composition having an advantageous combination of characteristics not readily attainable from competitive materials.
Heretofore prior technologists have established that most thermoplastic polymers are insoluble in other thermoplastic polymers. Most attempts to mix pellets of different molding composition at the time they are fed to an extruder lead to extrudates having fracture lines at the boundaries amongst the different compositions. Plastic "alloys" which contain a plurality of thermoplastic compositions have been discovered, but represent a minority of the theoretical number of possible mixtures. As greater knowledge of attractive compatibilities of blends has accumulated through the decades, some basis has evolved for selecting plausible fields for research on thermoplastic blends. Because incompatibility remains the dominating rule, any discovery of a useful blend of different thermoplastics represents a pragmatic invention which could not have been predicted on the basis of previous publications.
In an effort to improve resistance to impact, rubber-modified plastics have been prepared by copolymerizing a mixture of maleic anhydride and styrene in the presence of rubber, as shown in U.S. Pat. No. 3,919,354. Blends of such rubber-modified plastic and polycarbonate resins are described in U.S. Pat. No. 3,966,842.
Example 7 of Elghani et al U.S. Pat. No. 3,882,192 describes a thermoplastic molding composition blended from 40 parts of polycarbonates of a dihydric phenol and a derivative of carbonic acid, 35 parts of polyvinylchloride, and 25 parts of a copolymer of styrene and maleic anhydride (88:12 molar ratio) having a Vicat temperature of 110.degree. C.
Maleimide, N-methyl maleimide, maleic diamide, bis(N-methyl) maleic diamide, and related compounds have been studied by scholars but have not been reported as used significantly in the tonnage production of copolymers. However such compounds have been copolymerized with styrene in research projects. Outstandingly advantageous properties have been discovered for copolymers of styrene and such compounds, conveniently designated as styrene-maleimide copolymers. Notwithstanding the superior characteristics of such styrene-maleimide copolymers, the market for such copolymers has been small enough that it has been efficient to produce such copolymers by copolymerizing styrene and maleic anhydride and thereafter treating the copolymer with methyl amine or ammonia to obtain such styrene-maleimide copolymers. DiGiulio U.S. Pat. No. 3,998,907 describes a method of preparing maleimide-containing copolymers by reacting amine or ammonia with particles of the copolymer comprising maleic anhydride under autogenous pressure at 125.degree. to 200.degree. C.
Notwithstanding the abundance of literature pertinent to polycarbonates and blends comprising polycarbonates, there continued to be a persistent demand for a polycarbonate blend suitable for molding items at a convenient temperature, and a failure by others to meet such demand satisfactorily.